In a series circuit:

• A. Devices connected in parallel so the current splits
• B. The ground is mandatory
• C. Devices connected end-to-end so that current has only one path along which to flow
• D. A diode is used

Answer: (C)

In a series circuit, devices are connected end-to-end so that current has only one path along which to flow. That single path means the same current travels through every component, and the total resistance is the sum of the individual resistances, with voltages across components adding up to the source voltage. The option describing end-to-end connections with a single path best matches this setup.

The other ideas describe different concepts: current splitting occurs in circuits with parallel paths, grounding is not a defining requirement of a series circuit, and using a diode is not a defining feature of series configuration.